Nitrogen dioxide (NO2)
Updated 30 April 2024
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Accredited Official Statistics
Air quality statistics in the UK, 1987 to 2023 - Nitrogen dioxide (NO2)
Updated 30 April 2024
1. Why measure NO2?
Nitrogen dioxide (NO2) is a gas that is mainly produced during the combustion of fossil fuels, along with nitric oxide (NO). NO2 can also be formed when NO reacts with certain gases in the atmosphere, such as ozone. These reactions take place very quickly and are reversible, so the two gases are referred to together as nitrogen oxides (NOx).
Short-term exposure to concentrations of NO2 higher than 200 µg/m3 can cause inflammation of the airways and may increase susceptibility to respiratory infections (WHO, 2013). NO2 can exacerbate the symptoms of those already suffering from lung or heart conditions. NO2 can also cause changes to the environment. Deposition of chemical compounds containing NOx to the environment can also cause significant long-term harm to sensitive habitats by increasing nitrogen concentrations in soil or water. When soil nitrogen levels increase, common plant species that thrive in these conditions can dominate habitats at the expense of other rarer species. This can result in major changes to plant community structure (e.g. affecting the composition and diversity of plant species), which affects associated animal species.
In addition, NO2 is a ‘precursor pollutant’, as it is involved in the formation of ozone. Ozone is a gas which is also damaging to human health and can trigger inflammation of the respiratory tract, eyes, nose and throat as well as asthma attacks. Moreover, ozone can have adverse effects on the environment through oxidative damage to vegetation including crops.
The Air Quality Standards Regulations (2010) require that the annual mean concentration of NO2 must not exceed 40 µg/m3 and that there should be no more than 18 exceedances of the hourly mean limit value (concentrations above 200 µg/m3) in a single year.
2. Trends in concentrations of NO2 in the UK
2.1 Annual mean concentrations of NO2 in the UK, 1990 to 2023
The NO2 index shows the annual mean, averaged over all included sites that had annual data capture greater than or equal to 75%. The shaded areas represent the 95% confidence interval for the annual mean concentration for roadside sites, urban background sites and rural background sites. The intervals narrow over time because of an increase in the number of monitoring sites and a reduction in the variation between annual means at monitoring sites for NO2. Annual means for individual sites can be found in the NO2 statistical tables which accompany this release.
Figure 1: Annual mean concentrations of NO2 in the UK, 1990 to 2023
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Urban background NO2 pollution has reduced both in the long-term and in recent years
The annual mean concentration of NO2 at urban background sites was 14.2 µg/m3 in 2023, the lowest point in the time series. This is a decrease of 9 per cent since 2022.
Between 1992 and 2002 inclusive, the annual mean NO2 concentration at urban background sites rapidly decreased by an average of 2.7 µg/m3 each year. This reduction was observed at most monitoring sites across the UK and could be a consequence of the large reduction in emissions of nitrogen oxides over this time period in the UK and in Europe.
Between 2002 and 2006 the annual mean concentration fluctuated with no clear trend, and this was observed at most monitoring sites across the UK. Emissions of nitrogen oxides in the UK were still decreasing over this period but these years also coincided with increased use of coal in power stations, and with the increasing popularity of new diesel cars (which produce greater nitrogen oxide emissions than petrol equivalents).
Between 2006 and 2019 inclusive, the annual mean NO2 concentration at urban background sites decreased by an average of 0.9 µg/m3 each year. Reductions in concentrations were observed at most monitoring sites across the UK. Emissions of nitrogen oxides in the UK have continued to decrease as newer road vehicles subject to stricter emission standards enter the fleet and power generation moves away from the use of coal.
In 2020, the annual mean NO2 concentration at urban background sites decreased by 4.5 µg/m3 (a decrease of 23 per cent) compared to 2019. It is likely that a reduction in traffic as a result of COVID-19 restrictions was a contributing factor to this relatively large decrease.
In 2021, concentrations showed an increase of 5 per cent from a low point in 2020. It is likely that the lifting of some COVID-19 restrictions in 2021 was a contributing factor to this increase. Since 2021, concentrations have fallen each year to reach the lowest point in 2023. It is likely that this is at least partly a continuation of the declining trend seen since 2006 as emissions from road vehicles and power generation have continued to fall.
Roadside NO2 pollution has reduced in the long-term and in recent years, having been stable for most of the 2000s
The annual mean concentration of NO2 at the roadside has decreased over the time series to 21.8 µg/m3 in 2023. The annual mean NO2 concentration in 2023 is greater at roadside sites compared to urban background sites. This is most likely due to substantial emissions of nitrogen oxides from road transport sources, as the majority of concentrations at the roadside come from local road traffic.
For most of the 2000s, the annual mean NO2 concentration was relatively stable, likely as a result of the increased ownership of diesel-fueled vehicles which emit far more nitrogen oxides compared to equivalent petrol-fueled vehicles. This may have offset the impact of reduced emissions from other sources.
Between 2006 and 2019 inclusive, the annual mean NO2 concentration at roadside sites decreased by an average of 1.8 µg/m3 each year. This reduction was observed at most long-running monitoring sites across the UK and could be a consequence of the large reduction in road transport emissions of NO2 over the same period in the UK, as newer vehicles subject to stricter emissions standards enter the transport fleet.
In 2020, the annual mean NO2 concentration at the roadside decreased by 8.2 µg/m3 (a decrease of 26 per cent). It is likely that a reduction in traffic as a result of COVID-19 restrictions was a contributing factor to this relatively large decrease. From 2020 to 2021, the annual mean NO2 concentration at the roadside increased by 1.8 µg/m3, an increase of 8 per cent. This is likely a result of increased road traffic following the removal of lockdown restrictions.
Since 2021, NO2 concentrations at the roadside have fallen each year to reach the lowest point in the time series in 2023. Concentrations in 2023 are lower than they were before and during lockdown restrictions, with 2023 concentrations (21.8 µg/m3) being 30 per cent lower than 2019 levels (31.1 µg/m3). As for urban background, this is likely at least partially attributable to the continued decline in NO2 emissions from road transport and power generation.
Rural background NO2 pollution has reduced at a gradual rate over time and is at low concentrations generally
The annual mean concentration of NO2 at rural background sites has shown a decrease of 71 per cent from 1997, to 5.2 µg/m3 in 2023. Since the start of the time series in 1997, the annual mean NO2 concentration at rural background sites has decreased by an average of 0.5 µg/m3 (or 3 per cent) each year. This reduction was observed at most monitoring sites across the UK; which could be a consequence of the large reduction in emissions of nitrogen oxides over the same period in the UK.
3. Average hours spent in ‘Moderate’ or higher NO2 pollution
This metric measures the annual trend in the number of hours on average that concentrations are recorded at levels that may have impacts on human health. For NO2, ‘Moderate’ air pollution (the level at which citizens who are vulnerable to the health impacts of air pollution are recommended to take mitigating action) is triggered when the latest hourly mean concentration is greater than 200 µg/m3. Urban background sites rarely record concentrations at these levels so we have not included a graph for this and NO2 pollution was low at all urban background sites throughout 2023. The coloured categories relate to the categories of the Daily Air Quality Index (see Table 20 in the statistical tables that accompany this release).
Figure 2: Mean hours when NO2 pollution was moderate or higher for roadside sites, 1997 to 2023
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Roadside monitoring sites initially recorded a decreasing trend in the number of hours with ‘Moderate or above’ NO2 air pollution, until 2002. From 2003 to 2008 NO2 levels were significantly higher, which coincided with increased use of coal and a peak in the use of diesel cars. Since 2008 levels have fallen, which coincides with a reduction in the consumption of coal and a switch away from diesel vehicles.
For PM10 and PM2.5, ‘Moderate’ and higher air pollution has historically been experienced at many monitoring sites across the UK, but for NO2 ‘Moderate’ air pollution has been experienced by only a few particular sites. The overall trend follows the same trend as the London Marylebone Road site which recorded a peak of 853 hours in the ‘Moderate’ or higher categories in 2005 and has recorded at most 1 hour in this category in each year since 2019.
‘Moderate’ NO2 pollution at the roadside is usually the consequence of a build-up of emissions of nitrogen oxides from road transport sources, and the presence of street canyons or low wind conditions preventing dispersion of pollutants. For example, the London Marylebone Road site is on a six-lane congested road surrounded by high-rise buildings in central London.
4. Temporal variations in concentrations of NO2 in the UK, 2023
4.1 Weekday variations
The NO2 index shows the weekday mean, averaged over all included sites that had data capture greater than or equal to 75% for all instances of that weekday in a given year. The shaded areas represent the 95% confidence interval for the weekday mean concentration for roadside sites, urban background sites and rural background sites.
Figure 3: Weekday mean NO2 concentration at roadside, urban background and rural background sites, 2023
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NO2 concentrations tend to be lower at the weekend compared to during weekdays, particularly for roadside sites. This is likely to be primarily driven by less road traffic at the weekends. In 2023, the Monday-to-Friday mean concentration at roadside sites was 23.1 µg/m3, which was 18 per cent greater than the mean concentration at the weekend of 19.5 µg/m3.
4.2 Hourly variations
The NO2 index shows the hourly mean, averaged over all included sites that had data capture greater than or equal to 75% for all instances of that hour in a given year. The shaded areas represent the 95% confidence interval for the hourly mean concentration for roadside sites, urban background sites and rural background sites.
Figure 4: Hourly mean NO2 concentration at roadside, urban background and rural background sites, 2023
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NO2 concentrations tend to be much greater during the morning and evening rush hour compared to other times of the day for roadside and urban background sites. This is most likely due to commuter traffic. This pattern of concentrations follows the distribution of road traffic by time of day [footnote 1].
5. Sections in this release
Background to concentrations of air pollutants
Concentrations of particulate matter (PM10 and PM2.5)
Days with ‘Moderate’ or higher air pollution (includes sulphur dioxide)